She has a mouth infection. I am fighting the same issue with my Jackson's right now. Only a vet can treat this. The vet will clean out the mouth, try to drain some of the pus, and prescribe an antibiotic.
My recent vet visit cost about $60 USD, but it will depend on where you are.
Don't wait to see if it "gets better" and don't fuss about the cost of a vet visit. This could kill her if she doesn't get treatment.
Best of luck to your little lady. She's a pretty one.